The experiments described in this proposal will address the translation and RNA replication functions encoded by the 5' noncoding region (5' NCR) of picornavirus genomic RNAs. These functions are the result of protein-protein, RNA-protein, and RNA-RNA interactions involving both viral and cellular components. The proposed experiments will address the roles of viral RNAs and cellular proteins as they pertain to the separate activities of translation initiation and RNA synthesis. However, because there is functional overlap between some of the components utilized during cap-independent translation initiation by picornaviruses encoding type I IRES elements and proteins involved in picornavirus RNA synthesis, some of the proposed experiments will converge on both processes. Experiments outlined in this proposal will define the domains in the cellular RNA-binding protein, poly(rC) binding protein (PCBP2), required for multimerization and/or RNA binding and how such domains contribute to viral functions in the cytoplasm of infected cells. Such experiments will address important mechanistic questions regarding the role of PCBP2 and other cellular polypeptides in picomavirus translation initiation. This proposal will also test a hypothesis posited to explain a putative switch between translation and RNA replication on the same template (or templates) during a picornavirus infectious cycle. Finally, in vivo cell culture experiments are proposed that utilize recently- described experimental approaches to modulate protein levels and domain functions of cellular proteins hypothesized to participate in specific steps in the viral life cycle that involve the genomic RNA 5' NCR. Results from these proposed studies will provide new mechanistic insights into the complex processes involved in picornavirus translation initiation and into the interplay between translation functions and viral RNA replication. In addition, data generated from such studies should reveal fundamental aspects of RNA- protein interactions relevant to cellular cap-independent translation as well as pre-mRNA splicing and other elements of RNA metabolism.